Interest in integrated antennas on dielectric surfaces continues to grow as knowledge of construction techniques advances in the fields of microwaves and millimeter waves. This interest is particularly acute in applications demanding light-weight, low-cost antennas of simple construction with broadband capacity and narrow beamwidth. Examples of such applications include satellite communication antennas, remote sensing antennas, and radio telescopes for radio astronomy.
Implementations involving broadside radiating elements, such as dipoles, microstrip patches and slots are generally unsuitable for the above-mentioned applications in which gain of 10 dB and beamwidths of 12.degree.-60.degree. are typically required. That is, in free-space applications with satellites and the like, it is often desirable to employ antennas which can produce a symmetric endfire beam with appreciable gain and low side lobes. Endfire traveling-wave antennas, having moderately high directivity (10-17 dB) for a given cross section, are well suited for the above-noted free space applications.
One known example of an endfire traveling wave antenna is the flared slot-line antenna. A complete discussion of the flared slot-line antenna can be found in Prasad, S. and Mahapatra, S. A Novel MIC Slot-Line Antenna, Proceed. of Europ. Microwave Conf., Brighton, England (17-20 Sept. 1979). As disclosed by Prasad et al., the operation of a slot-line antenna is based on the fact that a slot-line begins to radiate as the slot is widened (i.e. flared). In one arrangement of a flared slot-line antenna, metallization is provided on a single side of a dielectric substrate to form a flared slot, and a microstrip feed-line is disposed along an opposing surface of the substrate to define a coaxial line transition from the feed line to the notch antenna slot line.
In another arrangement of the slot-line antenna, a blade antenna element, having metallization provided on a single side of a first dielectric substrate to form a flared slot, is positioned orthogonally to a ground plane which is underlaid by a second dielectric substrate. A microstrip feed-line is disposed on an opposing surface of the second dielectric to define a coaxial line transition.
Typically slot-line antennas, such as the two arrangements described above, include an open circuit between the antenna element and feed line such that impedance matching of the antenna element to the feed line is required. The open circuit places a limitation on the ratio of high to low frequencies that the slotline antenna device can properly receive or transmit. Moreover, improper impedance matching can generate discontinuities which limit the bandwidth of an antenna structure.
Monser et al. U.S. Pat. No. 3,836,976 discloses a dipole array of slot antennas, each of which has a narrow slot region and a wide slot region with the transition being made in a single step. More specifically, first and second conductors of a coaxial feed line are soldered to first and second metallization layers disposed on a substrate, respectively. Such metallizations are disposed in an opposing, spaced manner to define a slot therebetween on each antenna element. Consequently, a potential can be developed between the first and second metallizations to achieve transmission from the slot. In the preferred embodiment of the Monser et al. dipole array, pairs of orthogonal antenna elements are arranged such that each of the antenna elements are short circuited by a common, orthogonal ground plane, and pairs of coaxial feed lines are specifically positioned to define phase centers on the dipole array pairs. It should be appreciated that the components of the Monser et al. dipole array are arranged to allow for transmission in one of a variety of polarizations. While Monser et al. note that the feed lines could be of microstrip construction and that the slots could be flared from a narrow portion to a wide portion in other than one step, there is no teaching as to how a design incorporating such features could be conveniently or operatively achieved.
Nester, U.S. Pat. No. 4,500,887 discloses a notch antenna element, including a planar substrate having topside and bottomside metallizations, as well as a microstrip transmission line connected to one end of the antenna element. The metallizations define, in overlapping relationship, a two-sided slot line and a symmetrical two-sided notch antenna. Edges of the metallizations are shaped according to a function to facilitate smooth transition from the connection of the microstrip feed line to the symmetrical two-sided flared notch antenna.
In applications requiring radiation of energy over a relatively long distance, such as satellite applications, it is desirable to confine the radiated energy into a small area, i.e. to limit beamwidth, so that radiated energy can be minimized. That is, with most antenna arrangements, less power is required to achieve suitable signal level at a remote receiver if beamwidth is reduced. It has been found that symmetric endfire beams with appreciable gain and low sidelobes can be achieved in flared notch antennas by use of a "Vivaldi" configuration of a metallization on a dielectric substrate of suitable thickness. The characteristics of the Vivaldi type radiator are discussed in considerable detail in Yngvesson et al., Endfire Tapered Slot Antennas on Dielectric Substrates (IEEE Trans. on Antennas and Propagation, v. AP-33, No. 12, Dec. 1985). As indicated in the Yngvesson article, with proper configuration of the Vivaldi radiator and adjustment of the planar dielectric substrate thickness, beamwidth is frequency-independent over a considerable range of frequencies.
While the above-noted background reflects advances in the field of slot-line antennas, there is a need in the field of long-distance communication, such as satellite applications, for an easily fed, flared-notch antenna that is both simply and compactly constructed, and that substantially alleviates impedance matching concerns associated with feed components. More specifically, a need exists for a slot-line or flared notch antenna which achieves these desirable characteristics, while providing broadband performance capability and relatively narrow beamwidths. Additionally, the antenna should be lightweight as well as easy to both manufacture and install.